During the formation of paper from a pulp suspension it is of extreme importance from the point of view of the properties of the paper being produced that the paper web be formed in the wet section of the machine under controlled conditions.
Normally, the stock is ejected in the form of a free jet from the head box onto the wire, where the stock is dewatered and a paper web is formed. Formation of the web is influenced by many different potentially disturbing factors, such as e.g. incomplete dispersion of the fibers in the stock, non-uniform flow of the pulp out of the head box, differences in rate between the pulp jet and the wire, non-uniform dewatering due to unsuitable or deficient dewatering members, etc. It is particularly difficult to cope with the first two of these noted disturbing factors.
For geometrical-mechanical reasons, the fibers have a tendency to flocculate. This tendency towards flocculation is accentuated in connection with increasing fiber concentrations and fiber lengths. In order to be able to make a paper having a good formation, the fiber flocks must be well dispersed in the stock. This can be achieved by a very low fiber concentration, but in most cases that is not attractive, generally because it thus necessarily requires the handling of large amounts of flow. A degradation of the fiber flocks can also be effected by means of a fine turbulence in the stock flow. Machine manufacturers, therefore, have tried to establish flow geometries in the head box which yield a low-scale turbulence of sufficient intensity.
Based upon experience gained in practical operation, however, it is realized that this has created a dilemma. The turbulence thus generated often has a relatively wide spectrum, i.e. turbulence of relatively high scale mixed with turbulence of relatively low-scale. While the low-scale turbulence decays rapidly, and thereby gives rise to rapid re-flocking, the large whirls, which are rich in energy, have a longer duration, and thus often participate in the flow out of the head box. When the turbulence level in the jet ejected from the head box is too high, a change of the jet geometry (which originally is determined by the geometry of the slice) then results. The thickness of the stock jet shows local variations in time and geometry across the machine. As the substance of the sheet being formed depends upon the thickness of the stock layer across the wire, this substance thus varies from one position in the paper web to another.
These problems, which often relate to insufficient deflocculation of the stock, since the necessary turbulence level would unacceptably disturb the web formation on the wire, become considerably more serious in a fourdrinier machine than on a double-wire machine. On the latter machine, the free jet length is generally short, and dewatering takes place rapidly, so that thickness variations in the stock layer do not have sufficient time to grow to the same extent as they would in a fourdrinier machine.
Dewatering to a fixed state of the individual fibers in a fiber bed on a fourdrinier machine is carried out by different types of dewatering members, forming tables, table rolls, foils, and web suction boxes. In addition to their primary objective of dewatering, each of these members has a common element in that, to greater or lesser extents, they introduce disturbances into the stock layer. Dewatering by means of foils may thus be seen as an example of same. Foil strips are positioned at a predetermined angle relative to the wire, so as to form a diverging space with the wire in the direction of the machine. When the wire with the stock layer moves rapidly over the strip, a vacuum is formed in the diverging space, which brings about the dewatering. Greater or lesser amounts of the drained water follows along with the wire to the next foil strip on the lower side thereof, and at the front edge of that next foil strip the water is scraped off. This scraping off of water gives rise to a pressure pulse, which is directed upward against the wire, and to the web already formed and lying on the wire. The size of this pressure pulse is a function of the amount of water scraped off, the scrape angle, and the wire speed. For these reasons already mentioned, a state of flocculations which is unacceptable for paper formation exists in the stock flowing out of the head box. The pressure pulses produced at the front edge of the foil strips introduce shear stresses into the stock on the wire, and these stresses create a positive deflocculation effect at an early state in the web formation. This effect, however, is difficult to control, and if the pressure pulses are too strong at a somewhat later stage in the web formation the fiber network already formed on the wire can be destroyed, thereby negatively affecting the web formation.
Various methods and constructions have been proposed for solving these problems. It is known, for example, to apply a slice on a head box in such a manner that an upper slice lip extends forward over the wire in its direction of movement and over a dewatering member located beneath the wire. The intention of this is to establish a converging space adapted to the dewatering rate between the upper lip and the wire, so that a constant stock flow can be maintained in this space. During the greater part of the dewatering process, this results in a stock layer which is well-defined by the extended upper lip and the wire, and in which hydrodynamic disturbances generated in the head box have no possibility of developing. The converging space between the extended upper lip and the wire can have a shape which is defined such that the upper lip is stiff and the wire is supported by a dewatering member, providing the wire with a given tension. The dewatering member may be a suction breast roll or a plane suction box (whose appearance may vary). The open area in the cover of the suction box may be a pattern of holes or slits extending transversely to the machine. All suction box covers, however, have the open area and land areas arranged so that the wire is supported in a manner which implies a minimum of deflection in the suction zones. Suction boxes can be divided into sections, so that varying vacuum levels can be applied in the different sections. By this arrangement, the dewatering rate can be controlled so as to be adapted to the converging forming space. However, as mentioned before with reference to foils, a support beneath the wire during a dewatering phase implies that pressure pulses are directed upward against the wire, and a degradation effect on the web formed can again result. The situation is additionally aggravated by the fact that the fiber network formed is also not exposed to stabilizing suction forces above the land areas.
It is therefore an object of the present invention to eliminate, to the greatest possible extent, the aforesaid drawbacks arising in connection with paper web formation.